Various additives have been known to increase flowability (otherwise termed “slump”) in cementitious compositions, such as mortar and concrete, without increasing the water content of the initially formed composition. Such additives, or “admixtures” as they are also called, are classified as “water reducers” or “superplasticizers” when used for this purpose. One of the most commonly used of these water reducers are lignin-sulfonate compounds, also called “lignosulfonates,” obtained from sulfite pulping processes wherein cellulose is extracted from wood.
The so-called sulfite pulping process involves mixing sulfur dioxide (SO2) with an aqueous solution of base to generate the raw liquor for cooking the wood. In water, the sulfur dioxide forms sulfurous acid (SO2+H2O<--->H2SO3) which degrades and eventually sulfonates the lignin by replacing a hydroxyl group with a sulfonate group, allowing it to be solubilized and separated from the cellulose in non-precipitated form. The result is called “spent sulfite liquor” (SSL) and contains lignosulfonate and sugars, primarily monosaccharides, that need to be removed or destroyed so as to permit the lignosulfonate to be used effectively as a water-reducing concrete additive. Otherwise, high sugar levels accompanying the lignosulfonate can significantly retard the setting of the concrete to the point at which substantial delay of initial set time outweighs the water reduction advantage. If not removed, sugars are usually destroyed (e.g., through degradation, decomposition, etc.) and confer no appreciable benefit to the operation of lignosulfonate in concrete.
One of the objectives of the present invention is to provide compositions containing a lignin and/or lignosulfonate by converting, into useful acid or salt form, the aldose sugars that are present in agricultural residues (e.g., plant, tree, and other cellulose-containing materials). The present inventors believe that aldonic acids have excellent cement dispersing capabilities, and can therefore “beneficiate” the function of lignosulfonates, in particular, as water reducing additives.
Another objective of the present invention is to avoid, during recovery of lignin or lignosulfonate from agricultural materials, the very step of removing or destroying sugars and the attendant expense. Removed sugars are used as sweeteners. The sugars are typically removed by yeast. Sugars are otherwise destroyed through alkaline oxidation into small organic acids and carbon dioxide and water. Instead of removing or destroying the sugars, however, the present inventors propose to convert them, using enzymatic or microbiological means that are environmentally friendly and efficient. The sugars in the crude lignosulfonates can be aldoses, such as pentose and hexose.
Another and no less significant objective of the present invention is to convert aldopentose sugars into compositions that are rich in aldopentonic acids (or their salts), and, in particular, compositions that are rich in xylonic acid. Other exemplary compositions further comprise an aldohexonic acid or its salt (e.g., gluconic acid/gluconate). The present inventors surprisingly discovered that xylonic acid (or its salt form), which may be obtained through oxidation of xylose sugar, provides lower set retardation than gluconic acid or gluconates, which previously were known as water reducers in the concrete industry, at equal dosage levels. A preferred composition of the invention comprises a converted sugar in a ratio (aldopentonic acid/salt to aldohexonic acid/salt by weight) of 20:1 to 1:10.
A still further objective of the present invention is to provide a process that, while producing aldonic acid-beneficiated lignosulfonate-containing compositions, can be used for adding sugars from other sources (e.g., corn syrup, molasses), so that these can be converted as well into useful byproducts for modifying one or more properties of a hydratable cementitious material.